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Cetane Number is indicator of quality of lighter distillate fuels. It is an inverse function of a fuel’s ignition delay i.e. higher the cetane no. quicker the fuel will burn.
Accurate measurements of the cetane number are rather difficult, as it requires burning the fuel in a rare diesel engine called a Cooperative Fuel Research (CFR) engine, under standard test conditions. The operator of the CFR engine uses a handwheel to increase the compression ratio (and therefore the peak pressure within the cylinder) of the engine until the time between fuel injection and ignition is 2.407ms. The resulting cetane number is then calculated by determining which mixture of cetane (hexadecane) and isocetane (2,2,4,4,6,8,8heptamethylnonane) will result in the same ignition delay. The above engine was not designed for burning residual fuel, thus cetane no. cannot be calculated for residual fuels.
CCAI and CII are used instead for which formula is given. CCAI normally is 800~880. Lower the value, better the ignition quality. CCAI above 880 is unusable in diesel engine.